Ringactuators are piezoceramic stacks with a center bore. This design
allows additional features in mechanical, optical and electronic properties compared to
bulk stacks.

Piezo-ringactuators are available in available in monolithic low
voltage design and discretely manufactured high voltage versions. The actuators are
offered with preloaded stainless steel casing ("VS"-label) as well as in
"bare" versions without casing. Mechanically preloaded actuators are
preferentially used when

Acceleration forces during dynamic operation are applied to the
actuator.

Electronic Supplies, Power Requirements
Piezoelectric stacks show a distinct electrical poling. The stated maximum voltage has
only to be applied with the right electrical polarity. The "bare"
stacks without casing are completely electrically insulated ("potential free")
so both positive and negative supplies can be used by proper choosing the ground for one
of the leads. For ringactuators with casing VS, the electrical polarity is fixed and is
positive within the standard product range of actuators and power supplies/amplifiers. So
a free combination of actuators and supplies is achieved to optimize the power
requirements. Ringactuators with casing VS can be set to negative polarity on request
(without additional charge).N.B.: piezoelectric stacks expand, when the operating voltage of correct polarity
is increased. A voltage with opposing polarity can be applied to some extent
(approx. up to 10% of max. voltage rating). Stackactuators show then a contraction. This
effect can be used to increase the total displacement range of the actuators or to
generate an oscillation by a bipolar signal generator.

Electrical Capacitance: Piezoelectric actuators behave
like an electrical capacitor for driving frequencies well below their resonant
frequencies. The stated capacitances in the data sheet are valid for room temperature. Due
to variations in materials composition, the individual capacitances may vary by +15%.
The dielectric constant of the PZT materials varies with temperature and increases up to
50% for a temperature rise up to 80oC.

Optimizing the Power Requirements

For the operation of piezoelectric actuators, electrical power
(current) is only needed for a change of position or force. For highly dynamic
applications, the power requirements can be rather high resulting in high efforts and
costs for the supplies. Further on, the actuators warm up by dynamic operation due to
dissipative processes, converting some percent of the energy input to heat. These aspects
show clearly the need for power reduction/optimization techniques for dynamic operation of
actuators.

Power Matching Within An Piezo-Actuated System

Reduction of Strain - Usually, one tries to maximize strain by
applying the maximum operating voltage to the actuator, to get maximum effect from an
actuator with a distinct length. A short look shows, that this is preferentially a good
strategy in quasistatic applications with no or low dynamics (low operating frequencies
and/or stoke) and low power levels. For highly dynamic applications, the situation
changes. A high strain means nothing else as a high energy content which has to be
supplied by the electronics and is stored in most cases as reactive power. Then it might
be reasonable to use a longer actuator with a voltage below the maximum voltage rating,
leading to a reduced strain for the same stroke compared to a short stack.

A ringactuator CTC-HPSt500/15-8/15 (max. stroke 15
mm at 500 V) is used to produce a 5mm-oscillation
together with a 150 V supply. This arrangement needs only 1/3 of the
power compared to a CTC-HPSt500/15-8/5 actuator (stroke 5mm
at 500 V) together with a 500 V supply for the same
operating conditions.

Bipolar Operation - A further reduction of power
requirements is given for a bipolar operation of an actuator around zero position compared
to a one-side operation due to an unipolar driving signal for the same motion
characteristics. For this strategy attention has to be paid to depolarizing effects due to
countervoltage (for common actuators max. 10% counter voltage, higher ratings for hard
piezoceramics).

Resonant Systems - Resonant actuators systems show
the highest power efficiencies in converting electrical to mechanical energy compared to
broadband application under the conditions of a forced motion. The reason is, that
electronics has only to supply the effective power to the system and not the potentially
large reactive power resulting in a minimized selfheating. The efficiency in producing a
distinct oscillation is defined by the quality factor of the resonant systems. By
definition, these resonant systems are working on discrete frequencies and are not
broadband.

Piezomaterials Composition - There exist a variety
of formulations of PZTs with differing properties such as strain, power dissipation,
quality factor, cohercive field strength (stability against electrical depolarization).
Depending on the individual applications, power matching can be optimized by the proper
choice of the PZT-material. Please contact us for detailed consulting.

Positioning Sensitivity of Piezoelectric actuators - The piezoelectric
effect is unlimited in positioning sensitivity, there exists no principle "smallest
step". Piezocontrolled Scanning Tunnel microscopes (STM) or Atomic Force Microscopes
(AFM) are dealing with structures in Angstrom and Sub-Angstrom-range. In practice, the
positioning sensitivity is only limited by the stability and noise of the electronic
supplies.

Mechanical PropertiesNotice: Piezoceramic material and stacks can be damaged by tensile
forces.
The design of piezoactuated mechanical structures should avoid tensile stress for the
actuators. Actuators should be preferentially operated under "preload
conditions" (applying a permanent compressive load). Tensile forces are not only
produced externally, but can be generated inside as acceleration forces due to dynamic
operation (e.g. together with powerful supplies or electronic switches). Use
preferentially preloaded actuators for highly dynamic applications.Do not discharge a piezoelectric actuator by simple short-circuiting.

Stiffness: (inverse compliance, spring constant of the actuator) The
stated data for the actuators stiffness are defined for a load of approx. 10% of the max.
load.

Mechanical Resonance Frequencies
Like any other mechanical structure, a piezoelectric actuator shows characteristic
resonance's depending on actuators size, materials, manufacturing techniques, mounting
conditions, and externally coupled masses etc. Resonance's are not only observed for
the axial movement of the actuator, e.g. there exist radial modes for diameter
oscillations of the stacks. In positioning applications, such resonances may cause
problems e.g. for a feedback controlled system, if a resonance is within the operating
frequency band, it is not possible to get a stable position. So it is a main task in
designing such positioning arrangements, to ensure a sufficient high frequency of the
basic modes of the structure so not to interfere with the operating frequencies. Normally,
the lowest resonances of an actuator are axial modes, special cases with lower radial
modes are indicated. The datasheet states the lowest axial mode without externally
coupled masses and the actuator onside fixed on one side to a support, the other side
moving freely. The coupling of additional masses leads to a decrease of the
resonant frequencies of the system, compared to the unloaded actuator. An estimate for the
resulting resonant frequency of a mass loaded actuator is given by the simple spring-mass
model, where the actuators stiffness is set as spring constant.

Selfheating of Actuators
Due to internal dissipative effects of the ceramic material, some percentage of the
supplied (active or reactive) electrical power will be converted to heat. For dynamic
operation e.g. of some hundreds of Hertz with maximum stroke, the upper temperature limit
of common actuators can be reached. Still higher dynamics then require power matching e.g.
by reducing strain according the statements on page 6.

Mounting InstructionsAcuators without casing:
"Bare" ringactuators have to be mounted purely by the front faces and never by
the circumference. Radial clamping damages the ceramic body resp. may cause improper
function due to the not well defined mounting reference. Mounting of "bare"
ringactuators by the front faces can be done by clamping, optionally available threaded
endpieces HAg or by using adhesives like epoxies or cyanoacrylates.

Actuators with stainless steel casing
The ringactuators CTC-HPSt are offered additionally with mechanically preloaded stainless
steel casings VS. Now clamping at the circumference is possible as well as by mounting by
the threaded endpieces. Additional screw caps are provided for easy mounting of mechanical
and optical components.

SAFETY INSTRUCTIONS

When designing a piezoactuated system, please pay attention to the potential high
voltages and currents within such an arrangement, which may cause danger to life and
health, when handled improperly. Only authorized personnel should deal with high power
setups. Piezoelectric actuators as well as eletrostrictive actuators are electrical
capacitors, which can store electrical charge and energy for a long period. For bigger
actuators, the amount of stored energy can be large. Prevent damaging peripheral
electronics by erroneously connecting charged actuators.
Notice, that piezoelectric actuators can be electrically charged even by changing the
mechanical load of an actuator or its temperature. Before starting any manipulation of an
actuator, switch off all supplies and discharge the actuator by using a resistor.